NEW YORK — After repeated food poisoning outbreaks linked to romaine lettuce, the produce industry is confronting the failure of its own safety measures in preventing contaminations.

The E. coli outbreak announced just before Thanksgiving follows one in the spring that sickened more than 200 people and killed five, and another last year that sickened 25 and killed one. No deaths have been reported in the latest outbreak, but the dozens of illnesses highlight the challenge of eliminating risk for vegetables grown in open fields and eaten raw, the role of nearby cattle operations that produce huge volumes of manure and the delay of stricter federal food safety regulations.

A contested aspect of the regulation, for example, would require testing irrigation water for E. coli. The Food and Drug Administration put the measure on hold when the produce industry said such tests wouldn't necessarily help prevent outbreaks. Additional regulations on sanitation for workers and equipment — other potential sources of contamination — only recently started being implemented.

FDA Commissioner Scott Gottlieb said he thinks the combination of rules, once fully in place, will make vegetables safer to eat.

"I don't think any one element of this is going to be the magic bullet," Gottlieb said.

Health officials say improved detection may make outbreaks seem more frequent. Still, that is intensifying pressure on growers and regulators to prevent, catch and contain contamination.

It's not yet known how romaine got contaminated in the latest outbreak.

The spring outbreak was traced to romaine from Yuma, Ariz. Irrigation water tainted with manure was identified as a likely culprit, and investigators noted the presence of a large animal feeding operation nearby.

Subsequently, growers in Arizona and California adjusted an industry agreement to expand buffer zones between vegetable fields and livestock. The industry says the change was in place for lettuce now being grown in Yuma, which hasn't been implicated in the latest outbreak. But Trevor Suslow of the Produce Marketing Association said there isn't consensus about the exact distances that might effectively prevent contamination.

He noted specific buffer zones aren't required by the new federal rules on produce safety.

"They look to the industry to determine what is the appropriate distance," Suslow said.

Growers in Yuma also started treating irrigation water that would touch plant leaves with chlorine to kill potential contaminants, Suslow said. But he said such treatment raises concerns about soil and human health.

The FDA warned against all romaine last week because it said it was able to identify it as a likely source early enough. The agency narrowed its warning to romaine from California's Central Coast after the produce industry agreed to label romaine with harvest dates and regions, so people know what's OK to eat.

Stephen Basore, director of food safety at a Florida romaine grower, said he expects more regulations and self-imposed industry guidelines.

"Anytime there is an issue, the immediate response is saying our protocols aren't enough," he said.